Definition
A function of the en route air traffic control computer system that automatically alerts the controller when a tracked IFR aircraft is at, or is predicted within a set time to be at, an altitude below a predetermined minimum safe altitude for the area it is flying through.
Plain English
A computer safety net in en route air traffic control. It watches each aircraft's altitude and warns the controller if the aircraft is flying too low — or is about to be too low — for the terrain or obstacles in that area.
Context Anchor
Pilots may see E-MSAW in FAA acronym lists or in discussions of air traffic control safety alerts during the cruise or route portion of a flight.
Derivation
The 'E' stands for en route, meaning the cruise phase between departure and arrival. 'MSAW' has long existed near airports (terminal MSAW); adding the 'E' marks the version that watches aircraft during the en route phase, away from the terminal area.
Why Pilots Care
It adds an automatic safety layer that helps prevent controlled flight into terrain or altitude deviations during en route flight phases.
Grounding Statement
If an aircraft is descending or flying too low for the terrain along its route, E-MSAW can alert the controller before the situation becomes critical.
Intuition Check
E-MSAW is not a cockpit warning shown directly to the pilot. It is an automated alert for air traffic control that may lead the controller to warn the pilot.
Example Sentence 1
An E-MSAW alert prompted the controller to instruct the aircraft to climb immediately to 9,000 feet.
Example Sentence 2
E-MSAW monitoring runs in the background during en route segments to catch any unexpected descent below safe levels.